1. Field of the Invention
The present invention relates to a signal processing apparatus and, more particularly, to a signal processing apparatus arranged to convert, modulate, or store an input signal by performing analog signal processing of the input signal.
2. Description of the Related Art
In a wide range of industrial applications, there have recently been demands for systems to perform pattern recognition, such as, feature extraction from a large amount of input data. If complicated recognition patterns must be processed, computers may be substituted for skilled experts who heretofore made decisions as to which recognition patterns are to be processed. However, complex programs must be developed for this purpose and the scale of its data base must be tremendously increased and hence the necessity for memories for data storage becomes a serious factor. In addition, the design of such apparatus becomes complicated and the manufacturing cost increases accordingly as expected.
In such a situation, it is desirable to develop an expert information processing system which does not completely depend upon binary arithmetic-logic operations widely used in modern digital computers and which incorporates a novel signal processing system, in addition one which can be adapted to high-grade image processing and enables reductions in programming and data storage requirements. In recent years, devices or architectures for computers which simulate the processing mechanism of a neural system of a living body have attracted attention as one type of expert information processing system. A typical example is an information processing apparatus which employs a system constituted by coupling of nonlinear oscillators.
For example, Japanese Patent Laid-open No. 61-127075 proposes to convert an object to be recognized into the frequency of an oscillator system so as to define the object. Japanese Patent Laid-open No. 60-134391 discloses a pattern frequency converting apparatus which comprises a system in which nonlinear oscillators are arranged in a two-dimensional binary plane, and the apparatus is designed to have the nonlinear oscillators correspond to the "1" bits of an input pattern to compute a frequency of the system, thereby converting the input pattern into a frequency. Japanese Patent Laid-open Nos. 61-127076 and 62-103773 (which are patent applications on which U.S. Pat. No. 4,710,964 is based) disclose pattern recognition apparatus. Japanese Patent Application Laid-open Nos. 61-127076 and 62-103773 propose an apparatus for performing pattern recognition in a manner similar to the memory process and the information processing process of the brain of a living thing by utilizing the two functions of a nonlinear oscillator circuit, that is, an information processing function based on interference, in which an oscillation mode before interference strongly influences an oscillation mode after the interference and a memory function in which an oscillation pattern which has undergone interference can be maintained thereafter. In particular, in such a pattern recognition apparatus, the complexity of software and hardware becomes extremely low, since it depends upon the category in which the object is to be recognized, thus the design of the apparatus becomes easy and the reliability of the apparatus becomes high.
The aforementioned signal processing employs nonlinear oscillators according to the related art and adopts the so-called "entrainment" method as a self-organizing basic mode for signal conversion. Entrainment is a phenomenon in which a plurality of nonlinear oscillators which are coupled to one another interact reciprocally to deviate their frequencies from their respective basic frequencies, so that the nonlinear oscillators oscillate reciprocally. The phenomenon of entrainment by interference between nonlinear oscillators which are coupled to one another is explained in, for example, "Entrainment of Two Coupled Van der Pol Oscillators by an External Oscillation" (Bio. Cybern. 51,325-333(1985)).
Signal processing which makes use of the "entrainment" of oscillation in the nonlinear oscillator system utilizes, as its basic operating principle, the nature in which oscillations whose phase difference is small tends to strengthen their amplitudes reciprocally, while oscillations whose phase difference is large tends to weaken their amplitudes reciprocally. In other words, in the aforesaid related arts, whether or not entrainment has occurred is determined on the basis of amplitude of the signal, and the result is obtained as a binary output. More specifically, in each of those related arts, a binary input signal is converted into the magnitude of phase difference between nonlinear oscillators, and whether or not entrainment has occurred is represented in a binary manner on the basis of a comparison between two states, one being represented by the amplitude of oscillation having increased after mutual interference, and the other represented by the amplitude of oscillation having decreased after mutual interference.
This type of oscillation phenomenon, that is, "entrainment" of oscillation is the result of mutual interference between oscillators which takes place because the phase of each individual nonlinear oscillator is inseparably related to the amplitude of the oscillation. In other words, the differences of the phase and amplitude of the oscillation before and after "entrainment" can have an analog relationship or some other relationship, but not merely a conversion relationship. Accordingly, the signal processing capability of the nonlinear oscillator system is not sufficiently utilized only by using the amplitude of oscillation of a nonlinear oscillator system on a binary basis only. The foregoing is the starting point from which is derived the problem to be solved by the present invention.
It is not the intent to dynamically change parameters defining the oscillation of a non-linear oscillator in the above-described conventional signal processing method. For example, two non-linear oscillators are connected with fixed parameters to mutually interfere, and the state of the oscillator system has been uniformly determined by the term of the states of the two oscillators and the fixed parameters.
These problems lead to a drawback, i.e., if information processing such as pattern recognition is carried out using nonlinear oscillators which are coupled to one another in the conventional manner aforementioned, the signal processing potential of the system is limited to a high degree. More specifically, according to the related arts described above, an input signal which originally contains a large amount of information is converted into two simple binary states. One state wherein the amplitude of oscillation is large and the other state wherein the amplitude of oscillation is small, by means of the entrainment phenomenon in the nonlinear oscillator circuit. As a result, amplitude and phase which can be used as a signal carrier containing various kinds of information are restricted to simple binary form.